Funding for work on this invention was received from the National Science Foundation ERC under Award No. CDR-84-21402. The U.S. government may have rights in this invention.
The present invention relates to fiber optic taps and more particularly to a passive optical tap utilizing cladding modes in a single mode fiber to optically couple the fiber to a photodetector.
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
There are many applications in which small amounts of light need to be tapped from an optical fiber in order to broadcast to a large number of nodes, including optical clock distribution in synchronous optical local area networks (LAN's) and in very large scale integrated (VLSI) multiprocessor systems. The utility of such a light tap is often measured, in part, by its range of tapping ratios and its excess loss. These parameters determine the maximum number of stations that can be concatenated before signal regeneration by a signal repeater is required.
Commercially available passive optical taps, such as fused couplers, often couple light from one fiber to another through a thin cladding barrier. Single-mode optical taps exhibit tapping ratios as large as 20 dB and excess losses on the order of 0.1 dB, but these excess losses are fixed at some value which is not related to the tapping ratio, so that total excess losses of all taps are related to the number of nodes N. These characteristics are only adequate for the series connection of approximately 100 (i.e., N=100) taps or nodes in a typical optical system without repeaters, limited primarily by sensitivity constraints of optical receivers.
Several arrangements have been proposed which directly tap core modes by removing cladding and bringing a high index medium into contact with the exposed core, but these have involved multimode fibers. In one case, a high index medium is brought into direct contact with the exposed core of a 200/230 um multimode fiber, resulting in a 40 dB tapping ratio and less than 0.1 dB excess loss.(1) In another case, a hole in the cladding of a 50/125 um multimode fiber is filled with a high index medium.(2)
No satisfactory arrangement presently exists for passively tapping single mode optical fiber without resulting in significant excess loss, thereby significantly limiting the number of taps in an optical system or fiber optic bus for a repeaterless system.